Metal is deposited on a semiconductor substrate as a thin film to form electrical interconnections between the devices formed in the substrate. It is desirable for the metal thin film to deposit on the substrate surface in a conformal fashion so as to reduce voids, such as gaps and pinholes. Voids tend to form at the interface between the metal thin film and the substrate.
Voids are undesirable in a completed device because, when the device is in use, the flow of electrons around the voids tends to enlarge the voids according to a phenomenon called electromigration. As the voids grow in size, and the metal pathway conducting the current is correspondingly reduced, the reliability of the integrated circuit decreases. In time, one or more voids may grow to the point that the electrical circuit opens, and the device fails.
Thin films exhibiting a high degree of conformity with the substrate, may tend to also have a high degree of internal stress. The internal stress of the thin film can be sufficient to warp the substrate. This creates problems during subsequent processing of the substrate. The stress in the thin film can also create problems with loss of adhesion between the thin film and the substrate (delamination), or cracking of the metal thin film.
It is an object of the present invention, therefore, to provide a method of metal deposition that produces a metal thin film having a high degree of conformity to the substrate. A desired method should also produce reduced numbers of voids at the interface between the metal and the substrate and possess a low degree of internal stress.